It is generally known that it is advantageous to use a tread pattern having a directivity in the tire turning direction so as to ensure a good drainage performance. Patterns shown in FIGS. 4 and 5 are known as such directive patterns having a good drainage.
On a tread face 11 for which the tire turning direction R is designated in one direction, as shown in FIG. 4, there are arranged: four circumferential grooves 12 which are formed laterally symmetrically with respect to the tire equator line CL; and lefthand and righthand oblique grooves 13 which are arranged at predetermined pitches in a tire circumferential direction T and which are so extended to shoulder end portions as to expand from the two inner circumferential grooves 12 to the reverse tire turning direction. The directive tread pattern is formed with a number of blocks 14 divided by those circumferential grooves 12 and oblique grooves 13.
Meanwhile, in FIG. 5, the lefthand and righthand oblique grooves 13 are replaced by V-shaped grooves 15. The apexes of the V-shapes are positioned outside the tire equator line CL when the tire is attached to a vehicle, so that a directive tread pattern asymmetric with respect to the tire equator line CL is formed on the tread face 11. By adopting this asymmetric pattern, the turning drainage performance is enhanced more than that of the pneumatic tire of FIG. 4.
In recent years, a major problem is caused by the passage noise of vehicles. Especially, pneumatic tires having an improved drainage performance, as described above, produce high passage noise caused by the vibration of the blocks.
It is known effective as a method of lowering the passage noise in the prior art that the vibration of the blocks themselves are suppressed by enlarging the individual blocks to enhance the block rigidity. If the blocks are enlarged, however, the groove area of the oblique grooves 13 or the V-shaped grooves 15 is reduced. Therefore the drainage performance intrinsic to the aforementioned directive tread pattern is lowered. Especially the drainage performance at the time of turning is lowered.
The passage noise is caused not only by the aforementioned vibration of the blocks but also by the air pumping sound due to the grooves of the tread pattern. This air pumping sound is generated by the pumping action that the air in the grooves is compressed and released when the tread face is grounded onto the road surface while running. As a result, the pumping sound can be lowered more as the groove volume, i.e., the groove area is reduced more.
If the groove area is reduced, however, a drop in the drainage performance is invited, as described above. If, therefore, the passage noise is to be improved in a pneumatic tire having a directive tread pattern, there arises a problem that the compatibility with the drainage performance is extremely difficult.
The tire noise includes the exterior noise of the aforementioned passage noise and the interior noise. This interior noise is the noise which is generated by the contact of the tread portion of the tire running on the road surface with the road surface. It is generally known that the interior noise can be reduced by the pitch variation that transverse grooves are arranged at variable small pitches in the tread face in the tire circumferential direction. This effect of the pitch variation is heightened more for the smaller division of the tread face by the transverse grooves. If the tread face is divided into small, on the contrary, the tread rigidity is lowered, increasing the exterior noise. As a result, there arises a dilemma that the improvements in the interior noise and the exterior noise cannot be compatible.